Sharply tuned bandpass filter

ABSTRACT

A sharply tuned bandpass RF filter is presented herein. The filter has an input port for receiving television RF signals and passing same to an output port for application to a load. The filter is tuned such that it exhibits the characteristic of passing only RF signals having frequencies within a given frequency channel out of a plurality of channels, all having the same frequency bandwidth while rejecting all other frequencies. The bandwidth of each channel is on the order of W MHz. The filter exhibits an amplitude response within a mandated mask such that when operated in an RF transmitter the amplitude of the response is attenuated uniformly about the center frequency of the given channel within a frequency range of about ±0.5W MHz to a maximum of about ±(0.5W+0.45) MHz and extending to an attenuated level of about −1 to −18 dB from the in-band power level.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 60/195,238 filed Apr. 7, 2000 and U.S. Provisional Application Ser.No. 60/236,225 filed Sep. 28, 2000.

BACKGROUND OF THE INVENTION

The present invention relates to radio frequency (RF) bandpass filtersand, more particularly, to a sharply tuned bandpass filter that may beemployed as a stand alone filter or may be employed as part of afilter-combiner system.

DESCRIPTION OF THE PRIOR ART

Television signals have traditionally been broadcast in an analog formatknown as NTSC. The Federal Communications Commission (FCC) is nowpermitting a new digital format known as DTV. The digital format ispresently in operation and the FCC has provided a transitional perioduntil the year 2006 during which the NTSC signals and the DTV signalswill both be transmitted. Thus, a station will simulcast both an NTSCsignal and a DTV signal. It is understood that the FCC has allocatedfrequency bands or channels wherein an NTSC signal will be adjacent aDTV signal and also that a DTV signal will be adjacent to another DTVsignal. In the United States, the channels are all 6 MHz wide, whereasin other parts of the world the channels are 6-8 MHz wide. Thediscussion presented herein is specifically directed to channels thatare 6 MHz wide although it is to be understood that the discussion maybe similar for channels that are up to 8 MHz wide.

When a DTV allocation is one channel below an NTSC channel, this isreferred to as the N−1 case. When the DTV allocation is one channelabove an NTSC channel, this is referred to as the N+1 case.

To prevent adjacent channels from interfering with each other, it hasbeen known in the past to employ bandpass filters and hybrid couplers,simply referred to herein as hybrids, to form bandpass filter-combiners.Bandpass filter-combiners are described in a four page article entitled,Adjacent Channel Combining for N+1 DTV Channel Allocations, in the ADCTechnical News Publication, dated Jul. 30, 1998 published by ADCTelecommunications, Inc. FIG. 1 herein is based on FIG. 1 in thatpublication and presents a bandpass filter-combiner employing a pair ofbandpass filters 10 and 12 interposed between hybrids 14 and 16. One ofthe input ports of hybrid 14 is coupled to a reject load 18 and theother input receives an RF signal A. Another RF signal B is supplied toa port on filter 16. The bandpass filters 10 and 12 are tuned to passsignal A but reflect signal B. Consequently, signal A enters the leftside port of hybrid 14 and the signal splits into portions which passthrough the bandpass filters 10 and 12 and thence into the hybrid 16 andcombine with signal B. Signal B entered the hybrid and then split withportions reflecting off the bandpass filters 10 and 12 and returninginto the hybrid to combine with signal A to provide a combined signalA+B at an output port of hybrid 16. The article points out that thefilter-combiner of FIG. 1 is not practical for use in an N+1 casewherein the DTV channel is above the NTSC channel. This is because theaural carrier in the N+1 case is only 0.25 MHz away from the upperchannel edge, and may have sidebands up to 120 kHz away. Thus, the guardband is very small.

The ADC article proposes a design for the N+1 case and this takes theform shown herein at FIG. 2 which is based on FIG. 5 in the ADC article.FIG. 2 herein shows a constant-impedance filter-combiner 20 incombination with a notch diplexer 22. The constant impedancefilter-combiner 20 is based on that illustrated herein at FIG. 1 and,consequently, like components are identified with like characterreferences. Signal A is replaced with a DTV signal and signal B isreplaced with a VISUAL signal from an NTSC signal. The AURAL signal hasbeen removed and, as will be seen, is re-inserted downstream. Thebandpass filters 10 and 12 are tuned to pass the DTV signal and reflectthe VISUAL signal. The output port will provide a combined signalincluding the DTV signal and the VISUAL signal. This signal is thensupplied to the notched diplexer where the AURAL signal is re-inserted.The notched diplexer is tuned to reject or reflect the AURAL signal andto pass the combined DTV and VISUAL signals obtained from thefilter-combiner 20. The notched diplexer includes a left hybrid 24having an input port for receiving the combined signal from thefilter-combiner 20 and another input port connected to a load 26. Thenotched diplexer includes a pair of aural notch filters 28 and 30 thatextend from the left hybrid 24 to a right hybrid 32. The AURAL signal issupplied to a port 34 of hybrid 32 and the AURAL signal is then splitand portions reflected back from filters 28 and 30 so that the AURALsignal combines with the DTV and VISUAL signals and the combined signalappears at an output port 36 for application to an antenna 38. It is tobe noted that most notched diplexers also include −3.58 MHz traps inseries with the AURAL notched filters, such as filters 28 and 30 in FIG.2, to provide attenuation of unwanted color difference products in thelower sideband. For simplicity, such traps have not been included inFIG. 2.

By splitting the NTSC signal into its VISUAL and AURAL components, theprior art presented in FIG. 2 requires that the AURAL signal be addeddown stream. Consequently, in order to handle the N+1 case, this type ofprior art requires a circuit having three inputs for receiving the DTV,VISUAL, and AURAL signals. Additionally, this circuit requires the useof four hybrids 14, 16, 24 and 32. It is desirable to reduce the cost ofthe equipment needed to handle an N+1 case.

Attention is now directed to FIG. 3 which illustrates another prior artcircuit for handling the N+1 case. This circuit includes only twohybrids 40 and 42 as opposed to the four hybrids employed, in thecircuit of FIG. 2. The left hybrid 40 has an input port connected to aload 41 and a second input port that receives a DTV signal by way of apower amplifier 44. The DTV signal splits as it enters the hybrid 40 anda portion of it is passed by a DTV bandpass filter 46 and anotherportion is passed by a DTV bandpass filter 48. These DTV portions arepassed by AURAL notches 50 and 52 and then enter the right hybrid 42. AnNTSC signal that includes both AURAL and VISUAL components is suppliedto a port on the right side of hybrid 42 and these signals split in thehybrid and the AURAL portions are reflected from the AURAL notches 50and 52 and are passed back through the hybrid and recombined and areapplied to an antenna 60. The VISUAL portions of the NTSC signal passthrough the AURAL notch cavities of the AURAL notches 50 and 52 and arereflected by the DTV bandpass filters 46 and 48 and return through theAURAL notches 50 and 52 and thence into the right hybrid 42 and combinewith the DTV and AURAL signals and supplied to the antenna 60. Whereasthe circuit of FIG. 3 only has two hybrids, it is to be noted that theNTSC VISUAL signal passes over the AURAL notches twice during theoperation adding a heat burden to the AURAL cavities. Any temperaturedrifting will-cause performance parameters to fall outside theequalization set up because there is no tracking adaptive system inNTSC. Also, it is to be noted that a notch diplexer type combiner asshown in FIG. 3 results in reduced DTV to NTSC chroma isolation.Additionally, the AURAL notch causes equalization problems for thetransmitter system because the notched bandwidth can cut back into theNTSC chroma region which causes additional high frequency videoequalization adjustments.

From the foregoing discussion of the prior art circuits of FIGS. 1, 2and 3, it is seen that an improvement is needed in the bandpass filtersthat are employed in these combiners. A more sharply tuned filter isneeded that will pass the DTV signal for a particular channel whilereflecting all other frequencies including an adjacent channel NTSCsignal or an adjacent channel DTV signal. The filter should exhibit anamplitude response within a mandated mask (such as the FCC mandatedmask). Such a sharply tuned filter can be used as a stand alone filterhaving deep levels of IMD sideband rejection at the channel edges foradditional FCC mask suppression. This enhances transmitter performanceso that the transmitter power may be increased substantially whilemeeting the requirements of the FCC mandated mask.

In addition to operating as a stand alone mask filter resulting in powerenhancement, such sharply tuned filters may be employed in a constantimpedance filter-combiner without the need for notched filters for theAURAL carrier signal and, hence, such a combiner may be employed in theN+1 case as well as in the N−1 case as well as for adjacent channel DTVsignals.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a sharply tunedbandpass RF filter is provided. The filter has an input port forreceiving digital television RF signals and passing same to an outputport. The filter is tuned such that it exhibits the characteristic ofpassing only RF signals having frequencies within a given frequencychannel out of a plurality of channels, all having the same frequencybandwidth while rejecting all other frequencies. The bandwidth of eachchannel is on the order of W MHz (wherein W may be on the order of 6 MHzfor the United States and on the order of 6 to 8 MHz for other parts ofthe world). The filter exhibits an amplitude response within a mandatedmask (such as the FCC mandated mask) such that when operated in an RFtransmitter the amplitude of the response is attenuated uniformly aboutthe center frequency of the given channel within a frequency range ofabout ±0.5W MHz to a maximum of about ±(0.5W+0.45 ) MHz and extending toan attenuated level of about −1 to −18 dB from the in-band power level.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects and advantages of the present inventionwill become more readily apparent from the following as taken inconjunction with the accompanying drawings wherein:

FIG. 1 is a block diagram illustration of a bandpass filter-combiner inaccordance with the prior art;

FIG. 2 is a block diagram illustration of another bandpassfilter-combiner in combination with a notch diplexer in accordance withthe prior art;

FIG. 3 is a notch type combiner also in accordance with the prior art;

FIG. 4 is a block diagram illustration of a sharp tuned filterconstructed in accordance with the present invention and illustrated incombination with an amplifier and an antenna;

FIG. 5 is a graphical illustration of amplitude with respect tofrequency and illustrating typical filter specifications for amplituderesponse and also including the FCC mask and the sharp tuned filter maskherein;

FIG. 6 is a graphical illustration of time in nanoseconds versusfrequency and illustrating typical filter specifications with respect togroup delay;

FIG. 7 illustrates an embodiment of the invention herein employing twosharply tuned filters and two hybrids employed for supplying anamplified DTV signal to an antenna for broadcasting;

FIG. 8 is a block diagram illustration of an embodiment of the inventionherein for performing adjacent channel combining of an NTSC signal and aDTV signal in an N+1 case or an N−1 case;

FIG. 9 is a graphical illustration of amplitude with respect tofrequency showing the operation of the embodiment in FIG. 8 for adjacentNTSC and DTV channels in the N+1 case;

FIG. 10 is a graphical illustration of amplitude with respect tofrequency showing the operation of the embodiment in FIG. 8 for adjacentNTSC and DTV channels in the N−1 case;

FIG. 11 is similar to that of FIG. 8 but employed for combining adjacentchannel DTV signals;

FIG. 12 is a graphical illustration of amplitude with respect tofrequency showing the operation of the circuitry in FIG. 11 with respectto adjacent DTV channels;

FIG. 13 is another embodiment of the invention employing adaptiveequalization RF sample feedback;

FIG. 14 is another embodiment of the invention similar to that asillustrated in FIG. 13, but including additional features;

FIG. 15 is a graphical illustration of amplitude with respect tofrequency that show the lower adjacent channel response in an N+1 case;

FIG. 16 is a graphical illustration of amplitude with respect tofrequency illustrating the DTV bandpass amplitude response;

FIG. 17 is a graphical illustration of time in nanoseconds with respectto frequency illustrating the group delay response of the sharp tunedfilter;

FIG. 18 is a graphical illustration of amplitude with respect tofrequency showing the video response of the filter herein;

FIG. 19 is a graphical illustration of time in nanoseconds with respectto frequency illustrating the video group delay of the sharply tunedfilter the reduction in the equalization requirement of the transmitter;

FIG. 20 is a graphical illustration of amplitude with respect tofrequency showing improved isolation;

FIG. 21 is a graphical illustration of amplitude with respect tofrequency illustrating the monaural amplitude response of a sharp tunedfilter;

FIG. 22 is a graphical illustration of amplitude, with respect tofrequency showing a stereo signal amplitude response of the filterherein;

FIG. 23 is a graphical illustration of time with respect to frequencyshowing a stereo signal group delay response of the filter herein;

FIG. 24 is a graphical illustration of amplitude with respect tofrequency showing a typical transmitter spectral spread of a DTV signal;

FIG. 25 is a graphical illustration similar to that of FIG. 24 butshowing the 0.5 MHz FCC integrated power zone;

FIG. 26 is a graphical illustration similar to that of FIGS. 23 and 24but illustrating the operation of a transmitter employing a sharp tunedfilter constructed in accordance with the invention herein;

FIG. 27 is similar to FIGS. 24-26 but illustrating an enhanced highpower operation of a transmitter without a sharp tuned filter and with asharp tuned filter constructed in accordance with the filter describedherein;

FIG. 28 is a block diagram similar to FIG. 4 but including a feedbackpath and an exciter;

FIG. 29 is an elevational view illustrating a waveguide that may beemployed in practicing the invention herein;

FIG. 30 is a view taken along line 30—30 looking in the direction of thearrows in FIG. 29 and illustrating the configuration of the input portof the waveguide;

FIG. 31 is a view taken along line 31—31 looking in the direction of thearrows in FIG. 29 illustrating an iris plate;

FIG. 32 is a view taken along line 32—32 looking in the direction of thearrows in FIG. 29 illustrating another iris plate;

FIG. 33 is a view taken along line 33—33 looking in the direction of thearrows in FIG. 29 and showing a still further iris plate;

FIG. 34 is an enlarged perspective view of the filter that alsoillustrates adjustment screws (probes); and

FIG. 35 is a sectional, view showing iris plate 111.

DESCRIPTION OF PREFERRED EMBODIMENTS

Reference is now made to FIG. 4 which illustrates a sharp tuned filter(STF) 100 constructed in accordance with the present invention and whichis illustrated as having a single input port 101 and a single outputport 102. The input port is illustrated as being connected to the outputof an amplifier 104 which, for example, may receive and amplify adigital DTV signal for a particular channel, such as channel 10. The DTVsignal is applied to the sharp tuned filter 100 which is tuned to passonly signals in this channel, i.e. channel 10 in the example beingdescribed. The passed DTV signal is then provided at the output port 102and forwarded to a load, such as an antenna 106 for broadcastingpurposes.

It is to be understood that the load may take a form other than anantenna. The filter 100 may take various forms such as a length ofcoaxial cable or a wave guide. The structural configuration may takethese or other forms so long as the filter complies with thespecifications noted below and given with reference to the graphicalillustrations in FIGS. 5 and 6. For example, the filter 100 may take theform of waveguide 101 shown in FIGS. 29-33. This waveguide is a hollowcylinder constructed of suitable conductive metal, such as aluminum,copper or steel. The waveguide may have multiple cavities, such ascavities 103, 105 and 107 defined by iris plates 111, 113, and 115. Theiris plates and the three sections of the waveguide may be held togetheras with welding or bolt and nut arrangements. The waveguide has an inputport 117 and an output port. The input port is of rectangular shape, asindicated at FIG. 30. The iris plates have iris openings of variousshapes. For example, the iris opening in plate 111 is a cross 121 asshown in FIG. 31. The iris opening in plate 113 is a horizontal slot M45as shown in FIG. 32. The iris opening in plate 115 is a vertical slotM60. As best shown in FIGS. 34, 35, the waveguide 101 also has moderesonance screws i-vi and mode coupling screws M12, M34 and M56 thatprotrude into the cavities. These screws (or probes) are made of metal,such as copper, and extend into the cavities by various amounts, such asin the range from 0.25 inch to 0.50 inch for a circular waveguide havingan inner diameter on the order of 20.0 inches. Note that cavity 103,isfor mode resonance numbers 1, 2 and that cavity 105 is for moderesonance numbers 3, 4 and that cavity 107 is for mode resonance numbers5, 6. The cross, 121 includes slot M14 that couples resonances 1 and 4and slot M23 that couples resonances 2 and 3. The horizontal slot M45couples resonances 4 and 5.

The filter sections are realized thorough distributed capacitances andinductances through the use of the circular waveguide cavity sectionsseparated by conductive iris plates. The size of the cavities is suchthat two modes (such as modes 1, 2 or 3, 4 or 5, 6) are allowed topropagate through the waveguide (as opposed to the propagation of onlythe dominant mode).

The filter is tuned during construction to comply with thespecifications set forth in FIGS. 5 and 6. The tuning includes varyingthe length and inner diameter of the waveguide and varying the shape andsize of the iris openings and the orientation of the openings relativeto each other. The waveguide may be on the order of 6 feet in length andabout 20 inches in inner diameter. Additional tuning during filterconstruction involves adjusting the screws (probes) i-vi and M12, M34and M56. For example a first adjustment may be made as to the depth thatthese screws protrude into the waveguide cavities. Then a test maybemade by applying RF energy of the frequency channel of interest and thena determination is made with suitable meters (such as a spectrumanalyzer) as to the filter response relative to the FCC mask. This stepmay be repeated until the response meets the specifications herein. Theadjustment of the mode resonance screws i-vi change the resonantfrequency in the associated cavity. The filter bandwidth is changed byadjusting the three mode coupling screws M12, M34 and M56. Deeper screwpenetration provides greater mode coupling. This increases filterbandwidth. The opposite adjustment decreases filter bandwidth.

In FIG. 5 there is illustrated, in dotted lines, the FCC mask 110 forDTV signals sometimes known as the 8VSB standard or the 8VSB modulatedRF signal. The Federal Communications Commission (FCC) has mandated thateach television channel have a bandwidth of 6 MHz whether the channel bea DTV channel or an NTSC channel. The FCC mask 110 requires that allsignals broadcasted have their power attenuated starting at frequenciesno greater than ±3.5 MHz relative to the center frequency F_(c)of theassigned channel. The attenuation is complete. The FCC mask, as shown inFIG. 5, requires that the attenuation be continuous within the mask. Themask has left and right skirts 112 and 114 that extend in a linearfashion from mask edges 116 and 118 from the in-band power level 120 to−64 dB at ±9 MHz relative to the center frequency F_(c). The in-bandpower level 120 will sometimes be referred to herein as the referencelevel.

The filter 100, in accordance with the present invention, complies withand falls within the mandated FCC mask as is indicated herein by thesolid line 130 representing the filter mask of filter 100. This showsthe amplitude response of the filter. The vertical dashed lines 140 and142 represent the 6 MHz bandpass from −3.0 to +3.0 MHz relative to thecenter line frequency F_(c) that must be passed by the filter.Attenuation of signals beyond ±3.0 MHz up to about ±3.45 MHz, asindicated by the dashed lines 144 and 146, is achieved by the filter100. This attenuation is uniform about the center frequency extendingdownward to an attenuated level as indicated by the horizontal dashedline 148 and this attenuated level is at about −1 to −18 dB from thein-band power level 120. From this attenuated level, the amplituderesponse is further attenuated in a skirt like fashion to a level 150 ofabout −40 dB to −64 dB at ±9 MHz relative to the center line frequencyF_(c).

There may be some amplitude ripple at the in-band power level 120however this should stay within a response window 152 and not exceedabout −0.5 dB below the in-band power level 120. Additionally, theinsertion loss 154 should not exceed about −0.20 dB from the in-bandpower level 120.

Reference is now made to FIG. 6 which presents a graphical illustrationof time with respect to frequency showing the group delay as representedby curve 200 within the 6 MHz bandpass as represented by vertical dashedlines 202 and 204 at −3.0 and +3.0 MHz relative to the center frequencyF_(c). Points A and B are taken at −2.69 and +2.69 MHz relative to thecenter frequency F_(c)as indicated by the vertical dashed lines 206 and208. These lines 206 and 208 intersect curve 200 at points A and B whichare to be kept within 50 nanoseconds of each other.

The specifications of the filter 100 as presented in FIGS. 5 and 6 andas discussed above have been presented relative to the standards in theUnited States wherein the FCC has allocated television channels as being6.0 MHz wide. The European and other non-U.S. standards differ somewhatand, for example, the bandpass filter must be modified to passfrequencies on the order of 6 to 8.0 MHz which is the channel width orbandwidth in other parts of the world. Consequently, if the bandwidth ofeach channel is designated as being on the order of W MHz, then W may be6 for the United States and 6 to 8 MHz for other parts of the world.

The filter described herein with respect to FIGS. 4, 5 and 6 may beemployed as a stand alone mask filter with deep levels of IMD sidebandrejection on the channel edges. Such a stand alone mask filter isillustrated in FIG. 4 and the operation is described herein in greaterdetail with reference to FIG. 27. Such a stand alone mask filterenhances transmitter performance by reducing the linearity requirementsto meet the FCC mask and allows higher power transmitter operation forimproved power output. As such the filter provides greaterout-of-channel protection to guard against potential NTSC interferencewhich, in many cases, could be the same NTSC station due to currentadjacent channel allocations.

Additionally, the filter 100 as described relative to FIGS. 4, 5 and 6,may be employed in conjunction with another filter in a stand alonefilter mask in the manner as set forth in FIG. 7. There, a pair offilters 100A and 100B, of identical construction to each other and tofilter 100, are interposed between a left hybrid 300 and a right hybrid302. The filters 100A and 100B are tuned to pass the frequency channelof the DTV signal. The left hybrid 300 has an input port 304 connectedto a load 306 and a second input port 308 that receives an amplified DTVsignal from an amplifier 310. The DTV signal enters the hybrid 300 andis split into two paths, one extending through the filter 100A and theother passing through filter 100B. The two signals enter the righthybrid 302 where they combine and provide a DTV signal at the outputport 312 for application to an antenna or load 314. Another port on theright hybrid 302 is connected to a load 316. As will be discussed ingreater detail hereinafter with reference to FIG. 27, such a circuitwill easily meet the FCC sideband suppression levels while allowingoperation of the transmitter at a substantially higher power level. Thiswill be discussed in greater detail hereinafter with reference to FIG.27.

Attention is now directed to FIG. 8 which illustrates a constantimpedance filter-combiner 400 constructed in accordance with the presentinvention and employing a pair of filters 100C and 100D each constructedin the same manner as filter 100. Each filter 100C and 100D is tuned topass television RF signals within a particular channel while rejectingall other RF frequencies. In the example being presented, the mode ofoperation is for the N+1 case. Consequently, the DTV signal is from achannel of higher frequencies than the NTSC signal. For example, the DTVchannel may be that for channel 10 (192 MHz to 198 MHz) and NTSC channelmay be channel 9 (186 MHz to 192 MHz). In this example, both filters100C and 100D are tuned to pass the DTV signal (channel 10) whilerejecting all other RF frequencies. The DTV signal is supplied to apower amplifier 402 and, thence, to an input port 404 at the left sideof a hybrid 406. Another input port 408 of hybrid 406 is connected to areject load 410. The DTV signal enters the hybrid 406 at the input port404 and then is split into two portions which are respectively passed bythe filters 100C and 100D and enter a right hybrid 414 and arerecombined and are supplied to an output port 416. The NTSC signal,including both audio and video components, is supplied to a port 418 onthe right side of hybrid 414. The signal is then split in the hybrid 414and portions exit from the left side of hybrid 414 and are reflectedfrom the filters 100C and 100D and reenter the hybrid 414 and recombinealong with the recombined DTV signal and are provided at output port 416and applied to a road such as antenna 420. The hybrids in FIGS. 7 and 8may be zero degree or ninety degree hybrids.

The filter-combiner presented in FIG. 8 requires only two hybrids andonly two input ports (404 and 418) as opposed to the construction of theprior art filter-combiner in FIG. 2 that requires four hybrids and threeinput ports. Moreover, by providing such sharp tuned filters as filter100C and 100D, there is no need to provide AURAL notches such as AURALnotches 50 and 52 in the prior art filter-combiner of FIG. 3.

FIGS. 9 and 10 illustrate radiation patterns of amplitude with respectto frequency for the N+1 case and the N−1 case, respectively. Thus, inthe N+1 case as shown in FIG. 9, the NTSC signal is in the lowerfrequency channel, such as channel 9, and the DTV signal is in theadjacent higher frequency channel. This is a spectrum analyzerpresentation and it shows low DTV power which is normal.

The filter-combiner of FIG. 8 may also be employed in the N−1 case, asshown in FIG. 10, such as wherein the DTV signal is the lower channel,such as channel 10, and the NTSC signal is in a higher channel, such aschannel 11 (198 MHz to 204 MHz) Both FIGS. 9 and 10 show the radiationpatterns as measured and each division in the vertical directionrepresents 10 dB and each division in the horizontal directionrepresents 2 MHz.

Reference is now made to FIG. 11 which illustrates a combiner 400′ whichis virtually identical to combiner 400 and consequently like componentsare identified with like character references. The significantdifference is that this combiner combines two DTV signals, onerepresented as D1 which is applied through amplifier 402 to the inputport 404 of the left hybrid 406 in the same manner as discussedhereinbefore with reference to FIG. 8. In the case of FIG. 11 the DTVsignal D1 passes through the hybrids 406 and 414 and the filters 100Cand 100D as indicated by the dashed lines in the same manner as thediscussion presented above with reference to FIG. 8. The filters 100Cand 100D are tuned to pass only the DTV signals in the channel for thedigital signal D1 (channel 10 192 MHz to 198 MHz) and reject all otherfrequencies. The second digital signal D2 may be from an adjacentchannel which, is either higher or lower than that of channel 10. In theevent that it is of a higher frequency then it will be from channel 11.Since the filters 100C and 100D are tuned to reject such frequencies,the signal D2 will be split as it enters the right hybrid 414 and thesplit portions are reflected from filters 100C and 100D and pass backthrough the hybrid, as indicated by the dotted lines and combine withthe digital signal D1 for application to the antenna or load 420. Anamplitude versus frequency spectral plot for typical adjacent DTVchannels is illustrated in FIG. 12.

Reference is now made to FIG. 13 which is similar to FIGS. 8 or 11 inthat it employs a filter-combiner 400′ and may operate in an N+1, N−1 ora D+D configuration. As illustrated, a DTV transmitter 423 with anexciter 419 supplies a digital signal D1 to input port 404 and either anNTSC signal or a second DTV signal is supplied to input port 418 and theoutput is obtained from output port 416 and supplied to an antenna 420.An adaptive feedback path 417 provides feedback to the DTV exciter 419.A digital filter 421 at the input to the exciter 419 removes anyinterference that may be obtained from the NTSC signal or the second DTVsignal. The exciter includes pre-correction circuitry that pre-correctsthe information signal supplied to antenna to correct for any distortioncaused by the filters in the combiner-filter 400′. The pre-correctionmay be for both non-linear and linear distortions. Preferably thecorrection is for at least any linear distortion introduced by thefilters in the filter-combiner 400′. As shown, the correction may beadaptive with a feedback path. The correction may also be non-adaptive,without the feedback path.

Reference is now made to FIG. 14 which illustrates another embodiment ofthe invention similar to that with reference to FIG. 13 and which may beused when transmitting two DTV signals referred to herein as signals D1and D2. For purposes of illustration assume that D1 is the lowerfrequency signal and D2 is the higher frequency signal. This embodimentemploys a pair of exciters 500 and 502 which supply the D1 and D2signals to a pair of amplifiers 504 and 506. The amplified D1 and D2signals are supplied to a filter-combiner, such as combiner 400′ fromFIG. 11. The D1 signal is supplied to input port 404 and the D2 signalis applied through the filter 508 and thence to input port 418. Thefilter 508 is employed for passing the D2 signal and rejecting all otherRF signals and then the signal is applied to the filter-combiner. Theoutput signal which includes the combined D1 and D2 signals is suppliedfrom the output port 416 to the antenna 420. In this embodiment, asample is taken of the output signals D1 and D2 and fed back to theexciters 500 and 502 with the use of a signal splitter 510. Each of theexciters is provided with a filter to remove interference from thenon-related exciter. Thus exciter 500 is provided with a filter 512 toremove any interference from signal D2. Exciter 502 is provided with afilter 514 to remove any interference from signal D1. Both exciters 500and 502 include pre-correction circuitry that pre-corrects theinformation signal supplied to antenna to correct for any distortioncaused by the filters in the combiner-filter 400′. The pre-correctionmay be for both non-linear and linear distortions. Preferably thecorrection is for at least any linear distortion introduced by thefilters in the filter-combiner 400′. As shown, the correction may beadaptive with a feedback path. The correction may also be non-adaptive,without the feedback path.

FIGS. 15 through 23 illustrate many of the features of the responsecharacteristics of the filter described herein. These are specificallyrelated to operation of the embodiment illustrated in FIG. 8 whenoperating with adjacent channels in an N+1 case.

FIG. 15 illustrates the response curve between the NTSC and the DTVpassband zones. The sharp tuned characteristic is evident in the veryclose frequency spacing. Note the video frequency points marked as A, Band C. Point A is the chroma carrier at 3.58 MHz above the visualcarrier, point B is the video upper band edge at 4.2 MHz above thevisual carrier which is attenuated only −0.4 dB and point C is thereference visual carrier and shows little attenuation at the uppersideband frequencies around the chroma zone.

FIG. 16 illustrates the DTV bandpass showing useful in-band zonecovering an inner span of 5.9 MHz. The amplitude response just beyond5.9 MHz shows a rapid down turn in the response as the effect of thefilter becomes apparent. This is a very sharp roll off and is beneficialfor removing out of band IMD distortion products.

FIG. 17 is a graphical illustration of time with respect to frequencyillustrating a typical group delay response of the DTV bandpass of thefilter herein.

FIG. 18 is a graphical illustration of amplitude with respect tofrequency showing the video response of the filter relative to the loweradjacent channel (NTSC) and shows little attenuation at the uppersideband frequencies around the chroma zone.

FIG. 19 is a graphical illustration of time with respect to frequencyshowing the group delay characteristics of the lower adjacent channel.

FIG. 20 is a graphical illustration of amplitude with respect tofrequency and shows the improved isolation needed for satisfactorilycombining NTSC and DTV signals.

FIGS. 21, 22 and 23 illustrate audio performance aspects of the sharplytuned filter.

FIG. 21 is a graphical illustration of amplitude versus frequency andthe monaural bandwidth and which is expanded from that illustratedherein at FIG. 15.

FIG. 22 shows the stereo bandwidth.

FIG. 23 is a graphical illustration of time with respect to frequencyshowing the typical group delay characteristics of the audio path in thefilter-combiner herein.

FCC Mask Compliance

The FCC rules state, in part, “in the first 500 kHz from the authorizedchannel edge, transmitter emissions must be attenuated no less than 47dB below the average transmitted power”.

Reference is now made to FIG. 24 which illustrates a typical DTVtransmitter spectral spread known in the prior art. The total integratedpower between the vertical dotted lines 600 and 602 is the referencepower for determining if the −47 dB requirement is met in the adjacent0.5 MHz zones.

FIG. 25 illustrates the 0.5 MHz zone 604 of the typical DTV transmitterspectral spread illustration of the prior art. It is to be noted thatspectral spread illustration of FIG. 25 is without the employment of asharp tuned filter, such as filter 100 described herein with referenceherein to FIG. 4 as well as with reference to the other versionsillustrated in FIGS. 7, 8, 11, 13 and 14. To the contrary, theillustration in FIG. 25 is representative of the prior art and it isclear that this does not meet the FCC −47 dB requirement.

Attention is now directed to the graphical illustration in FIG. 26 ofamplitude with respect to frequency showing the spectral spread when thesharp tuned filter 100 described herein is employed. At the location ofthe FCC 0.5 MHz adjacent zone 604, the power is −51 dB below the in-bandpower level 610. Clearly, this shows FCC mask compliance as more thanthe −47 dB level has been met noting specifically location 612 in theillustration of FIG. 26.

Moreover, it is noted from FIG. 26 that the filter herein providessignificant reduction of out of band spectrum components.

Power Enhancement

Reference is now made to FIG. 27 which illustrates at curve 614, the IMD(intermodulation distortion) sideband suppression characteristic withoutemploying the sharply tuned filter herein. Curve 616 illustrates thesideband suppression characteristic with the sharp tuned filter hereinwhen an IOT transmitter is operating at about 30% above normal powerreadings (note that the in-band power level 610′ in FIG. 27 is about 30%greater than in-band power level 610 in FIG. 26).

Reference is now made to FIG. 28 which is similar to FIG. 4, butincludes an adaptive feedback path 700 and a DTV exciter 702. Theexciter 702 includes a pre-corrector circuit 704 that pre-corrects theinformation signal supplied to antenna or load 106 to correct for anydistortion caused by the filter 100. The pre-correction may be for bothnon-linear and linear distortions. Preferably the pre-correction is forat least any linear distortion introduced by the filter 100. As shownthe pre-correction may be adaptive with a feedback path. Thepre-correction may also be non-adaptive, without the feedback path. Thepre-correction should compensate for any linear distortion, such as anyripple in the in-band power level 120 in FIG. 5 caused by filter 100during such higher power operation noted in the waveform of FIG. 27.

Summation

-   1. A sharp tuned bandpass filter (STF) is a key element in a TV    combining system for adjacent channel allocations where special    techniques are required to combine very closely spaced channels.    Current industry techniques use a set of notch diplexing cavities on    the NTSC aural carrier in a constant impedance configuration to    combine adjacent, upper or lower, DTV channels with an NTSC channel.    The filter-combiner described herein at FIG. 8 uses a set of sharp    tuned filters to do a very effective job of combining the NTSC and    DTV signals with significantly enhanced performance on the NTSC.-   2. The NTSC aural performance is improved with nearly double the    bandwidth of the notch cavity diplexing approach. This avoids    sideband cutting of the “PRO Channel” and group delay and amplitude    distortion of the SAP and BTSC Stereo subchannels. The notch    diplexing approach introduces an “S” curve in the group delay    response which makes it very difficult to correct. In addition, the    amplitude response is sloped off in an asymmetrical manner further    complicating the overall correction of the aural carrier. The sharp    tuned filter herein eliminates “S” curve response problems with a    soft group delay response that in many cases doesn't need correction    or very little of it.-   3. Video performance is enhanced with very little amplitude    distortion and only mild group delay distortion which can be easily    corrected. This is unlike the notch diplexing system that requires    amplitude and delay correction circuits to restore the severe    amplitude roll off of the 4.2 MHz burst flag due to the notch    cavities. The sharp tuned filter system needs only modest group    delay correction to restore the 12.5 T pulse performance that can be    easily accomplished with existing analog circuits.-   4. The DTV to NTSC isolation characteristic is significantly    degraded in the notch cavity system which causes high frequency    bleed through of DTV components into the NTSC channel. The sharp    tuned filter system described herein does not have this problem and    provides a flat, uniform isolation value of about −35 dB over the    video band.-   5. The specified response and attenuation values shown herein    provide the necessary symmetry in the DTV channel for elementary    adaptive group delay and amplitude response correcting circuits to    achieve automatic adaptive correction without losing control. The    symmetry issue as specified is an important issue to reach the    standard accepted error vector magnitude for DTV transmission.-   6. The sharp tuned filter approach can combine adjacent channel DTV    signals.-   7. The attenuated point at +/−0.25 MHz off the upper and lower edge    of the filter provides additional attenuation to meet the FCC out of    band mask when the transmitter power is increased to a point where    the spectral spread shoulder level can be as great as −30 dB. Full    mask compliance can therefore be obtained when the DTV transmitter    is operated at higher than normal power levels for enhanced    efficient operation. The sharp tuned filter herein provides this    characteristic. The benefit of higher power operation while    maintaining FCC mask compliancy is a significant advantage.-   8. The sharp tuned filter system with the attenuation specification    shown herein exceeds the current FCC mask requirements and provides    further sideband reduction for noticeably improved interference free    operation on adjacent NTSC channels.-   9. The system design concept of using a sharp tuned filter system    for the unique application of adjacent upper or lower DTV channel    combining with an NTSC station, only recently allocated by the FCC,    with the performance as stated above.-   10. A sharp tuned filter system causes very little group delay and    amplitude errors over the video pass band and in particular, has a    very smooth response curve over the aural pass band to maintain good    BTSC performance and low synchronous AM.-   11. The filter-combiner herein can combine adjacent DTV channels. An    adaptive system can be connected to the system for adaptive    correction on each independent DTV path. A special filter in the    adaptive sample back line can be used in the exciter to filter out    the adjacent DTV interference for excellent dual DTV transmission.-   12. The filter herein, however, does restrict the 8VSB bandwidth    slightly which is a benefit for reduced adjacent channel    interference but does not cause any reduced performance on DTV    receivers. There is no measurable change in DTV receiver threshold    levels, hence no coverage loss. Transmitter EVM performance remains    excellent.

From the above description, those skilled in the art will perceiveimprovements, changes and modifications. Such improvements, changes andmodifications within the skill of the art are intended to be covered bythe appended claims.

1. A sharply tuned bandpass RF filter, said filter having an input portfor receiving digital television RF signals and passing same to anoutput port of said filter for application to a load; said filter beingtuned such that it exhibits the characteristic of passing only RFsignals having frequencies within a given frequency channel out of aplurality of channels all having the same frequency bandwidth whilerejecting all other frequencies; said bandwidth of each said channelbeing on the order of W MHz; said filter exhibiting an amplituderesponse within a mandated digital television signal mask such that whenoperated in an RF transmitter the amplitude of the response isattenuated uniformly about the center frequency of said given channelwithin a frequency range of about ±0.5W MHz to a maximum of about±(0.5W+0.45) MHz and extending to an attenuated level of about −1 to −18dB from the in-band power level.
 2. A filter as set forth in claim 1wherein W is in the range from about 6.0 MHz to about 8.0 MHz.
 3. Afilter as set forth in claim 1 wherein W=6.0 MHz.
 4. A filter as setforth in claim 3, wherein said mandated mask is the FederalCommunications Commission (FCC) mask for the 8VSB standard.
 5. A filteras set forth in claim 4 wherein said mandated mask requires that theamplitude response be at ±3.5 MHz about said center frequency at thein-band power level and extend therefrom in a skirt like fashion to alevel of about −64 dB at ±9.0 MHz.
 6. A filter as set forth in claim 5wherein the maximum amplitude response of said filter extends from saidattenuated level to a level of about −40dB to −64 dB at about ±9.0 MHzwithin said mandated mask.
 7. A filter as set forth in claim 6 whereinthe maximum amplitude response of said filter extends from saidattenuated level in a skirt like fashion.
 8. A filter as set forth inclaim 7 wherein said skirt like fashion varies in a linear manner.
 9. Afilter as set forth in claim 6 wherein said filter exhibits thecharacteristic of maintaining the amplitude of any variations in themagnitude of said in-band power level within about 0.5 dB.
 10. A filteras set forth in claim 6 wherein said filter exhibits the characteristicof maintaining any insertion loss within about −0.20 dB from saidin-band power level.
 11. A filter as set forth in claim 6 wherein saidgiven frequency channel has a channel edge and that said filter exhibitsthe characteristic of attenuating any emitted power within 0.5 MHz fromsaid channel edge by an amount greater than −47 dB below said in-bandpower level.
 12. A filter as set forth in claim 1 in combination with apower amplifier located upstream from said filter and a pre-correctorlocated upstream from said power amplifier to compensate for any signaldistortions that may be introduced by said filer.
 13. A filter as setforth in claim 12 wherein said pre-corrector is a pre-corrector thatcompensates for any linear in-band distortions that may be introduced bysaid filter.
 14. A filter as set forth in claim 1 in combination with apower amplifier and a DTV exciter that provides a DTV signal as saidgiven frequency channel for amplification by said power amplifier withthe amplified DTV signal being applied to said filter, said filterhaving an output port for providing a filtered amplified output DTVsignal.
 15. A filter as set for in claim 14 wherein said exciterincludes a pre-corrector for pre-correcting said DTV signal tocompensate for any distortions that may be introduced by said filter.16. A filter as set forth in claim 15 wherein said pre-correctorcompensates for any linear in-band distortions that may be introduced bysaid filter.
 17. A filter as set forth in claim 16 including a signalsampler that obtains a sample of said filtered amplified output DTVsignal, a feedback network that feeds said sample to said pre-correctorfor adaptively compensating for any distortions introduced by saidfilter.